AIR-PRESSURE FLUCTUATIONS IN A PRAIRIE SOIL

The influence of air convection on soil aeration has not been investig ated in great detail. A study was conducted to measure air pressure ch anges by depth in the first 2 m of a prairie soil that was known for i ts denitrification properties. An absolute air pressure probe was used to measure air pressure fluctuations at the soil surface and newly de signed differential air pressure probes were placed at depths of 0.1, 0.2, 0.3, 0.4, 0.5, 0.75, 1, 1.5, and 2 m. A procedure of in situ reca libration was used every 6 or 7 wk for these probes. At the same depth s, capacitive probes and thermocouples measured the volumetric water c ontent and temperature, respectively. The bulk density profile, water table level, and rainfall amounts were recorded. The atmospheric press ure ranged between 97 000 and 101 000 Pa during the experiment. The si gnal drift of the air pressure probes was negligible for four of the p robes and <50 Pa for the of hers. The accuracy of the measurements wit h signal drift compensation was superior to 10 Pa. Differences of air pressure between the soil at the surface and at depth may be >2000 Pa. These differences, which are not necessarily linked to the depth, are probably due to soil heterogeneities and generally to water infiltrat ion. Air pressure seemed to level oat quickly when the air-filled pore space was continuous. This generally prevented ''dephasing-amortizati on'' effects. When the air-filled pore space was discontinuous, differ ential air pressures depended simultaneously on the atmospheric pressu re and temperature fluctuations.